(1) Field of the Invention
The invention relates to power conservation. More specifically, the invention relates to reduction of power consumption in a low power mode.
(2) Background
Reduced power consumption is generally regarded as a desirable characteristic, but in some power critical applications, reducing power consumption may become essential. Certainly, in mobile devices reduced power consumption will increase battery life and permit longer operation in the mobile environment. Additionally, certain industry specifications require power consumption at a very low level. For example, the Universal Serial Bus (USB) Specification Version 1.0, published Jan. 19, 1996, mandates that bus powered devices draw no more than 100 milliamps after power on, 500 milliamps after enumeration, and no more than 500 microamps when the device is in sleep mode. As USB devices have become increasingly complex, the entire sleep mode power budget has been consumed by the central processing unit (CPU) in the USB device. This leaves insufficient power budget to supply other auxiliary circuitry, including, but not limited to, powering the memory for the processing unit, and any other functionalities.
The power problem is exacerbated by the fact that the CPU in sleep mode must be able to awaken rapidly in response to a wake-up signal. Thus, the CPU cannot be wholly disconnected from the power supply during sleep mode. Prior approaches attempting to permit compliance of USB devices with the sleep mode power budget have generally failed. One attempted solution is to have the CPU turn off the auxiliary circuit responsive to receiving the go-to-sleep signal from the bus. Unfortunately, this creates problems in fetching the sleep instructions, as the memory for the CPU is often included in the auxiliary circuitry. Moreover, a greater problem arises when the wake-up signal arrives and the auxiliary circuitry is not yet powered. As a result, the CPU is unable to fetch the instructions needed to appropriately respond as required. Other attempted solutions have generally failed to permit the devices to meet the 500 microamp sleep budget. This problem has been sufficiently vexing that many manufacturers have been lobbying the USB Standards Committee to increase the power budget for sleep mode of USB compliant devices.
In view of the foregoing, it would be desirable to provide a way to satisfy power budgets in extremely power-constrained systems, while still permitting acceptable response time and transitioning in and out of a low power mode.